The exhaust gas discharged by a diesel engine contains, for instance, particulate matter (PM) and nitrogen oxides (NOx). There are methods in which a filter (diesel particulate filter) having a catalyst or the like is provided in an exhaust passage of an engine, in order to capture PM in the exhaust gas, and fuel is injected into the exhaust passage, upstream of the filter, in order to remove the PM captured by the filter (see Japanese Patent Application Laid-open No. 2002-295298 and Japanese Patent Application Laid-open No. 2007-315313).
Means for injecting fuel into an exhaust passage include, for instance, injectors for low-pressure injection (for instance, multihole nozzles and swirl nozzles for gasoline engines). In driving control of such injectors for low-pressure injection, an actuator that drives the injector is driven at a constant driving frequency (for instance, 100 Hz). The amount of fuel injected by the injector is controlled by changing a duty ratio that is determined on the basis of the driving frequency.
The initial stage of valve opening in the injector comprises a so-called dead time period (duty ratio of about 0 to 10%) in which a needle valve (needle) is not lifted, and no fuel is injected. The last stage of valve opening includes a so-called stable time period (duty ratio 90% or higher) in which the needle valve is substantially not closed. Therefore, the injector is used within a duty ratio range of about 10 to 90%, by avoiding the dead time and stable time (see injection amount characteristics in FIG. 5 (with pressurization)).